FIGS. 1-3 hereof relate to a typical known or conventional transfer molding apparatus. As best seen in these figures, such a known transfer molding apparatus typically comprises an upper center block 2 of an upper metal mold, a top mold base 3 mounted on the top center block 2, and an ejector pin plate 4 fixed to a lower surface of top drive plate 5 which is located above the top mold base 3. The known apparatus also includes a relatively long cylindrical pot 6 through which a heated resin 8 is forcibly passed into a bottom center block 1 of a lower metal mold. The cylindrical pot 6 extends through the top drive plate 5, the ejector pin plate 4, the top mold base 3, and the top center block 2. A movable plunger 7 is provided for applying a force to the heated resin 8 by reciprocatively moving into an upper end of pot 6 above the resin 8.
As best seen in FIG. 1, the known apparatus includes chases 9, 9' mounted on opposite sides of the top center block 2, riser blocks 10, 10' mounted on the top mold base 3, and clamp blocks 11, 11' mounted uppermost for clamping the upper metal mold downwardly toward the lower metal mold.
In the conventional transfer molding apparatus per FIGS. 1-3, there are also provided one or more top cavity runner ejector pins 21, top center block runner ejector pins 22, top ejector cavity pins 23, top drive plate spring 24 and top return pin 25. See, in particular, FIG. 2.
The above-described known apparatus also includes bottom chases 30, 30', bottom cavity runner ejector pins 31, bottom drive plate spring 32, bottom mold base 33, bottom ejector pin plate 34, bottom drive plate 35, bottom cavity ejector pins 36 and bottom return pin 37. Note, in FIG. 2, the parting line at which the upper and lower mold dies meet and separate in the process of encapsulating small electrical elements in molded resin.
In general operation with such a known apparatus, after a lead frame comprising one or more small objects, e.g., electrical units, is loaded on a bottom mold die (not shown) which has been previously adequately heated, the upper and bottom mold dies are clamped to each other by operating the apparatus. Typically, the bottom mold die may be held stationary and the upper mold die may be pressed downwardly thereto. Upon clamping of the upper and bottom mold dies, the top cavity ejector pins 23, the top cavity runner ejector pins 21, and the top center block runner ejector pin 22, all of which are slightly projected from the immediately adjacent surfaces of the cavity and the runner through which they pass, are pushed upwardly by the top return pin 25 which is fixed to the top drive plate 5.
The ejector pin plate 4 is fixed to the top drive plate 5 by means of a bolt (not shown), and the top drive plate 5, and the ejector pin plate 4 all move together under a biasing force applied by the top return pin 25.
Upon completion of a molding cycle, and when the apparatus is opened, the top drive plate 5 descends under a biasing force provided by the top drive plate spring 24 which is mounted to the top drive plate 5. Also, the top cavity ejector pin 23 descends so that the molded product is separated from the molding dies.
In such a conventional prior art apparatus, the top drive plate 5 is provided as a single integral element having a thickness "t" through which the upper end of pot 6 must extend to its upper opening into which plunger 7 is forcibly introduced. An undesirable consequence of this is that an amount of air corresponding to the volume in pot 6 within a length corresponding to thickness "t" is entrapped between the bottom surface of plunger 7 and the top surface of heated resin 8 contained in pot 6. Plunger 7 then descends over the entrapped air into pot 6 to apply pressure to the heated resin 8 to inject it into the space defined between the top and bottom dies to surround and encapsulate the small electrical elements placed therein. The entrapped air is thus pressurized and becomes intermingled with the heated flowing resin to creating foaming and voids, etc. The result is that the electrical elements are not properly and uniformly encapsulated.
As persons of ordinary skill in the art will appreciate, such inadequate insulation of electrical elements can lead to highly undesirable electrical shorting under diverse operating conditions. This specific problem is addressed by the present invention, and the solution requires a relatively simple but highly effective structural change from the conventional prior art apparatus as described above. This structural improvement results in vastly improved products in which foaming, porosity and other known defects in the encapsulating cured resin are significantly reduced or substantially eliminated.